How to Test for Nan in JavaScript

How do you check that a number is NaN in JavaScript?

Try this code:

isNaN(parseFloat("geoff"))

For checking whether any value is NaN, instead of just numbers, see here: How do you test for NaN in Javascript?

How do you test for NaN in JavaScript?

The question has the answer if you read closely enough. That is the way I found it: I was typing out the question and... bingo.

You remember when I wrote "NaN is like NULL in SQL, it is not equal to anything, even itself"? So far as I know, NaN is the only value in Javascript with this property. Therefore you can write:

var reallyIsNaN = function(x) {
   return x !== x;
};

Parsing to Check if NAN Javascript

Checking if == NaN will always return false. The proper way to do this is with Number.isNaN():

var button = document.getElementById("button");

button.onclick = function() {
    var numberTest = parseInt(document.getElementById("numberTest").value);
    if (Number.isNaN(numberTest) || numberTest == "" || numberTest === null) {
        alert("No Number");
    }
    else {
        alert("This is a number!");
    }
};

You also had some issues with your logic in the if - each clause is a separate expression, and therefore needs to check against numberTest every time.

How do I test for NaN?

NaN's are unusual: they are not equal to anything, even themselves. You need to use isNaN(inbperr) to tell whether a value is a NaN or not.

Check if a user input is NaN

You need to use isNaN()

let hour = parseInt(prompt('Enter Hour'));

while (hour > 12 || hour < 1 || isNaN(hour)) {
    hour = parseInt(prompt('Enter Valid Hour'));
}

How to check if value is NaN in Typescript?

Same as JavaScript, isNaN.

if (isNaN(someObject.someValue)) ...

Or the more modern Number.isNaN.

if (Number.isNaN(someObject.someValue)) ...

The difference between the two is that isNaN() will coerce the tested value into a number before checking (values with typeof value !== 'number' will return true), while Number.isNaN() won't.

In other words, with these values, you get these results:

value	`isNaN()`	`Number.isNaN()`	`typeof value === 'number'`
`123`	`false`	`false`	`true`
`NaN`	`true`	`true`	`true`
`true`/`false`	`true`	`false`	`false`
`'a string'`	`true`	`false`	`false`
`new Date()`	`true`	`false`	`false`
`[]`	`true`	`false`	`false`
`{}`	`true`	`false`	`false`
...	`true`	`false`	`false`

i don't know how to use ' NaN ' in javascript's if command

The input age will be a string. Use isNaN() instead. This will work as you expected.

<!DOCTYPE html>
<html>
    <head>
    </head>
    <body>
        <!--Javascript starts-->
    <script>
        var age = prompt("How old are you ?","18");
           if(isNaN(age))
           {
               alert("its not a number");
           }else
           {
               alert("its a number");
           }  
    </script>
        <!--End of Javascript-->

    </body>
</html>